1. Field of the Invention
This invention relates to an automatic exposure control circuit for a photographic camera or the like and, more particularly, to an arrangement for including information on film sensitivity in an exposure control signal.
2. Description of the Prior Art
Heretofore, a camera of the kind having a so-called half-opening shutter in which the shutter blades are arranged to serve combined functions as a diaphragm and a shutter and in which an exposure control program is determined by a shutter opening time. Such cameras have been arranged in general to have film sensitivity information supplied to a light measurement computing circuit in an optical manner with a filter or aperture set in front of a light sensitive element in such a way as to correspond to the film sensitivity information set by the photographer. Known electrical methods for supplying the film sensitivity information have been as follows: When a photo conductive cell, such as a CdS cell is used as the light sensitive element, the film sensitivity information is supplied by shifting resistance, in the case of a camera using a photo diode and suppressing and which utilizes expanding a current produced therefrom, the film sensitivity information is supplied by shifting the voltage determining level of a capacitor which is arranged to be charged with an expanded current thus obtained. However, the film sensitivity information values are widely distributed. In order to cover all the film sensitivity values of the ISO system, many indexing steps are necessary. Arrangement to electrically shift such wide-ranged film sensitivity information at an increment or decrement of 1/3 step then results in an increased number of parts or in a voltage varying range broadened by 2.sup.n times (n: the number of steps of film sensitivity information) this results in increased size of the apparatus or a complex circuit.
The automation of the design of cameras has advanced year by year. In the case of the so-called lens-shutter camera, it is now only the arrangement for setting the film sensitivity which remains not automated. To attain this automation, the above-stated optical information supplying method necessitates a complex structural arrangement. Therefore, it has been thus desired to have a compact circuit which is capable of processing an electrical input and giving accurate exposure information against temperature fluctuations. There has also appeared a film cartridge of the kind arranged to be capable of indexing all of the 24 different film sensitivity values of the ISO sensitivity system by means of a five-place indexing part as disclosed in Japanese Patent Application Laid-Open No. SHO 57-104129.